1. Field of the Invention
This invention relates to image forming apparatuses.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a printer, copier, facsimile machine, or multifunction peripheral thereof, the toner amount (i.e. toner density) of a toner image formed on a photoconductor (e.g. photoconductor drum) directly affects on the quality of a print image. The toner density changes over time according to some conditions such as usage environment and usage time length. For instance, while keeping a developing bias of the photoconductor constant, changing an electrostatic charging characteristic due to its atmosphere may result in reducing toner supplied from a development unit to the photoconductor.
Since a cleaning blade for cleaning the photoconductor and an intermediate transfer member contact the photoconductor and grind its photoreceptor layer, increasing the number of times of image forming results in decreasing the thickness of the photoreceptor layer, and consequently surface electrostatic potential of the photoconductor is difficult to keep constant. Gradually decreasing the surface electrostatic potential results in gradually increasing the toner density of a toner image and degrading image quality.
In order to solve this problem, a toner image for density adjustment (hereinafter called “test pattern”) is carried on a predetermined area of the intermediate transfer member to which a toner image formed on the photoconductor is primary transferred, the density of the test pattern is measured with a sensor, and the toner density is adjusted by controlling process conditions such as developing bias according to the measurement result. For example, the test pattern is formed so as to contain areas with gradually different toner densities.
The reflectance of a surface material of the intermediate transfer member affects on the measurement result of the test pattern. Contacting a cleaning member, a transfer roller for secondarily transferring a toner image on a record medium such as a paper sheet and the like to the surface of the intermediate transfer member results in staining and scratching the surface and adhering a toner external additive to the surface. Therefore, a surface condition of the intermediate transfer member is measured with the sensor before forming the test pattern, and the toner density is adjusted with taking into account the detection value of the sensor.